A femto cell means an indoor base station for low power home use or office use. Although the femto cell is used as the same meaning as that of a pico cell, its function is more evolved than that of the pico cell. The femto cell is a small scaled cellular base station connected to a broadband router, and serves to connect audio and data of 3G as well as audio and data of 2G to a backbone network of a mobile communication provider through a digital subscriber link (DSL).
This femto cell has accelerated spread of 3G, and may be used as an initiator that increases indoor coverage. Also, it is expected that femto cell mobile station users will be increased to reach one hundred million people or more within several years and thirty million or more base stations will be provided. In view of technical aspect, indoor coverage enhancement of the technologies such as wideband code division multiple access (W-CDMA), high speed downlink packet access (HSDPA), evolution-data only (EVDO) is very important in providing services. As traffic is routing through IP network, network quality and receive capacity have been enhanced and at the same time operating cost invested by mobile communication providers for a backhaul dedicated line has been reduced. In this respect, the femto cell is very advantageous in view of strategic and economical aspects.
The femto cell is able to configure an independent network. Unlike the existing cells, the femto cell has a relatively small cell radius. FIG. 1 is a diagram illustrating a configuration of a femto cell. Referring to FIG. 1, a plurality of femto cells may exist in one macro cell. Supposing that a cell having a large radius is a macro cell in view of general aspect, the femto cell is a tiny cell that is connected with a general cell through backbone connection and performs communication by using its network capacity. The femto cell is different from a relay station (RS) in that the femto cell cannot be directly connected with another femto cell. Unlike the macro cell, this femto cell can be turned on/off, and can be operated in a sleep mode for power saving.
This femto cell can be divided into a closed subscriber group (CSG) and an open subscriber group (OSG) depending on features of an access terminal. Only an authorized mobile station can access the CSG femto cell, and all the mobile stations may access the OSG femto cell without any specific condition.
A legacy system is a system that follows the existing specification and corresponds to the related art system. For example, an institute of electrical and electronics engineers (IEEE) 802.16e system corresponds to the legacy system. However, the legacy system is not limited to the IEEE 802.16e system only. A new system more evolved than the existing system may be provided in a zone where the legacy system is provided.
There is provided a method for enabling a legacy system to scan a neighboring cell in a handover procedure. In case of the existing IEEE 802.16e system, a mobile station can receive neighboring cell information by requesting a base station of a scanning interval allocation request (MOB_SCN-REQ) message. Also, the base station may directly inform the mobile station of neighboring cell information by periodically transmitting a neighbor advertisement (MOB_NBR-ADV) message to the mobile station, whereby the mobile station can receive the neighboring cell information. However, when this neighboring cell search scheme is applied to femto cell search in the IEEE 802.16m system, several problems occur.
Unlike the existing macro cell, a femto cell newly introduced in the IEEE 802.16m may include many cells having similar power characteristics in case of similar frequency assignment. If many cells having similar power characteristics exist, the number of cells to be searched is increased, whereby overhead is increased. If the femto cells are operated at different FAs, the number of cells to be searched is more increased. Also, if the system supports a location based service (LBS), the number of femto cells to be searched can be limited to femto cells in the periphery of the mobile station. However, unless the LBS is supported, it is difficult to actually use the femto cells. Also, since the femto cell can be operated in an inactive mode unlike the existing macro cell, its list management is required.
An example of the femto cell search scheme according to the related art includes an inter-working signal (IWS) based femto cell search scheme. The femto cell is synchronized with the macro cell, and can transmit the IWS to the macro cell base station for FA of the macro cell. At this time, the femto base stations that use the same FA can transmit the IWS to the macro cell base station by using the same resource, while the femto base station that use their respective FA can transmit the IWS to the macro cell base station by using different resources. Afterwards, the mobile station can obtain information on neighboring femto cells by receiving and detecting the IWS from the macro cell base station while performing communication with the macro cell base station. In other words, the mobile station can detect the femto cell even without separate scanning for different FAs.
FIG. 2 is a diagram illustrating an example of an IWS transmitted from a femto cell base station to a macro cell base station during femto cell search according to the related art.
Referring to FIG. 2, as suggested, the IWS has coverage similar to that of a broadcast channel (BCH), and a resource allocation scheme is the same as illustrated in FIG. 2. For example, femto cells for two FAs can simultaneously be detected through a preamble signal divided by a frequency division multiplexing (FDM) mode at a system bandwidth of 5 MHz. At this time, a total of 288 data tones can be used.
FIG. 3 is a diagram illustrating examples of a type of an IWS transmitted from a femto base station to a macro cell base station.
A problem occurs in that it is difficult to identify a femto cell through preamble information only, which includes information such as cell index, from the IWS type illustrated in (a) of FIG. 3. A mobile station (MS) needs layer 2 (L2) information, which is media access control (MAC) information, from the femto base station before initiating handover (HO) or deciding to wake up the femto base station.
On the other hand, since femto base station ID is unique at the IWS type illustrated in (b) of FIG. 3, the mobile station (MS) doe not need L2 information from the femto base station before initiating handover (HO) or deciding to wake up the femto base station. However, since short femto base station ID is only a part of femto base station ID, a problem occurs in that it is difficult to detect the short femto base station ID once and that the short femto base station ID should be searched again.
In the aforementioned femto cell search scheme according to the related art, since pilot tones are divided into two types as illustrated in FIG. 2, two FAs or two femto base station IDs have been searched once. For this reason, a problem occurs in that the number of femto base stations is increased. Also, if the mobile station directly searches for the femto base station, a problem occurs in that much time is required to search for the femto base station.
A synchronization channel is used for physical synchronization or used to identify segment information and base station ID. In case of the IEEE 802.16e system, these two uses of the synchronization channel have been achieved using a preamble of one symbol. In case of the IEEE 802.16m system, these two uses of the synchronization channel have been achieved respectively by a primary synchronization channel (P-SCH) for physical synchronization and a secondary synchronization channel (S-SCH) for segment information and base station ID.
For example, if the synchronization channel is used for segment information and base station ID, three segments of 3k, 3k+1, 3k+2 (k=0, 1, 2, . . . , N−1, 3(N−1)=2 is smaller than or equal to the number of used subcarriers) are defined in a subcarrier unit, wherein base station ID can be identified by a code division multiplexing (CDM) mode using N number of orthogonal sequences per segment. In this case, segments may represent three sector IDs in case of three sectors.